The present invention applies to a semi-conductor optical amplifier insensitive to polarization of light and offering good performance. In particular, it has high gain, high signal-to-noise ratio and a high confinement factor.
Semi-conductor optical amplifiers, insensitive to polarization of light, are currently manufactured by a number of different methods. Along these methods, two in particular have been used recently because their theoretical results appeared promising.
A first method consists in growing on a substrate in III-V material, of InP type for instance, an active layer of rectangular section. The material of this active layer is furthermore chosen so that it is subject to a slight tensile stress. To this end, its mesh parameter is slightly lower than the mesh parameter of the substrate.
FIG. 1 is a diagrammatic representation of this active layer rectangular section. FIG. 3 shows the variation of the gain G (in dB decibels) of the active layer 1 of the amplifier depending on the various modes TE (in solid lines) and TM (in dotted lines) of polarization of the light, depending on the stresses .DELTA.a/a. The gain G of the active layer is proportional to the product g.times..GAMMA. where g represents the gain of the gain specific to the constituent material of the active layer, and .GAMMA. represents the confinement factor. In this method of production, the constituent material of the active layer is subject to slight tension. Typically, the tension .DELTA.a/a is of the order of -1.6.times.10.sup.-3.
However, a slight variation of the tensile stress .DELTA.a/a causes a large variation of the gain G of the active layer. On the curve of FIG. 3, it can be seen that if a variation in gain of less than a dB is desired, the variation in stress .DELTA.a/a must be less than 1.10.sup.-4. Now a degree of precision such as this of the tensile stress to be applied to the material of the active layer 1 is not easy to control. Furthermore, since the stress applied in the active layer 1 is a tensile stress .DELTA.a/a, the gain G(TM) of this active layer, in the TM mode of polarization of light, is greater than the gain G(TE) in the TE mode of polarization of light. It is therefore very difficult to control the variations in gain G of the active layer of the amplifier. It consequently appears very difficult to balance the gains G(TE) and G(TM) of the active layer by controlling the tensile stress.
Additionally, the thickness of the section of the active layer 1 plays an important part with regard to the confinement factors .GAMMA. since it enables the ratio .GAMMA.(TE)/.GAMMA.(TM) to be controlled. In the example in FIG. 1, this thickness is for example 300 nm, while the width w of the section is for example 1,000 nm. In fact, the rectangular shape of the section of the active layer enables a higher confinement factor .GAMMA. to be obtained in TE mode than in TM mode.
From the relationships G(TM)=g(TM).times..GAMMA.(TM) and G(TE)=g(TE).times..GAMMA.(TE), and the fact that the slight tensile stress creates a gain, in the material of the active layer, such that g(TM)&gt;g(TE) and that the shape of the active layer gives a confinement factor such that .GAMMA.(TE)&gt;.GAMMA.(TM), it suffices to adjust the shape of the section, and in particular its thickness, and the tensile stress in the material, to cause variations in the parameters g(TE), g(TM), .GAMMA.(TE) and .GAMMA.(TM) in such a way as to obtain equality between the gains G(TE) and G(TM) of the amplifier. This equality between the gains G(TE) and G(TM) of the amplifier makes it possible to make the amplifier insensitive to the polarization of light.
However, the thickness of the active layer is also very difficult to control with precision. Generally it is not possible to control an absolute thickness at better than 2%. This 2% margin of error therefore also leads to a margin of error, which cannot be controlled, in the confinement factors .GAMMA.(TE) and .GAMMA.(TM).
Since the variations of gains in the material of the active layer g(TE) and g(TM), and the confinement factors .GAMMA.(TE) and .GAMMA.(TM) are difficult to control, it follows that it is very difficult to balance the gains G(TE) and G(TM) in order to make the amplifier insensitive to the polarization of light. This first method does not therefore, in practice, enable satisfactory results to be obtained reproducibly.
A second method consists in growing on a substrate in III-V material, of InP type for instance, an active layer with a quantum well structure. A section of this active layer 10 is diagrammatically represented in FIG. 2. The quantum structure consists of a series of very thin layers with quantum wells 11,12, alternately stressed in compression 11 and tension 12, and barrier layers constituted by a material with a broad band of forbidden energy and a low refractive index.
In this case, the tensile and compressive stresses are high. The tensile stresses .DELTA.a/a are less than -6.10.sup.-3 and the compressive stresses .DELTA.a/a are greater than +6.10.sup.-3. In this case, the layers under tensile stress have a gain g(TE) inherent in the material of which they are constructed which is negligible by comparison with their gain g(TM). Conversely, the layers under compressive stress have a gain g(TM) inherent in the material of which they are constructed which is negligible by comparison with the r gain g(TE) The active layer of the amplifier constructed according Lo this second method thus has an alternation of very thin layers with quantum wells which are purely TE or purely TM, i.e. the layers with quantum wells each allow the propagation of only one of the modes TE and TM of polarization of light.
Since the overall gain G of the active layer of the amplifier is proportional to the product g.times..GAMMA., all that remains is to vary the confinement factors .GAMMA.(TE) and .GAMMA.(TM) of the different layers to balance the gains G(TE) and G(TM)in order to make the amplifier insensitive to the polarization of light. To this end it is necessary to adjust the number of quantum wells in the layers, i.e. it is necessary to control the ratio of the thicknesses of the different layers. The control of a ratio of thicknesses can be carried out with greater precision than the control of absolute thickness. It thus appears that the confinement factors can be adjusted more readily than in the case of the previously described method.
However, in this second previously known method of construction, the different layers are very thin, generally between 6 and 12 nm. This thinness of the layers means that low confinement factors .GAMMA.(TE) and .GAMMA.(TM) are obtained. Now it is important, in order to obtain high efficiency, that semi-conductor optical amplifiers should have high confinement factors. To increase the value of these confinement factors .GAMMA.(TE) and .GAMMA.(TM) it is necessary to multiply the number of quantum wells and of barriers 13. This multiplication, however, leads to other problems, in particular problems of an electrical nature, because the injection current becomes non-uniform between the wells.
Furthermore, to form the quantum structure of the active layer 10, it is necessary to grow three different types of material: a first type to form the layers with quantum wells under tensile stress 12, a second type to form the layers under compressive stress 11, and a third type to form the barrier layers 13. Now the process of growing three different types of material to form a succession of very thin layers is difficult to control with precision and complex to set up. Consequently, amplifiers manufactured using this second method do not always in practice give the hoped-for results.